The present invention relates generally to techniques for detecting the presence of radioactive scale in a well without running a log prior to the placement of radioactive tracers in the well. More specifically, the present invention involves a technique for using the gamma ray spectral signature of the scale to determine the position and quantity of the scale.
The art of pumping various fluids and solids downhole is well known. The objective is to place them in the formation so as to enhance stimulation and/or completion of a well. In many instances, the materials placed downhole are tagged with one or more radioactive isotopes that emit gamma rays. The radioactivity of these elements allows their presence to be detected and for this reason these isotopes are commonly referred to as tracers. The isotopes that are most commonly used as tracers are iridium (.sup.192 Ir), antimony (.sup.124 Sb), gold (.sup.198 Au), iodine (.sup.131 I), and scandium (.sup.49 Sc).
By tagging the materials that are pumped into the formation with one or more of these tracers, it is possible to monitor the placement of the materials using a gamma ray spectroscopy tool. Such a tool measures the gamma ray spectra at a sequence of depths along the borehole and uses the measured spectra to determine the locations of the individual tracers. The gamma ray data contain information regarding the radial distributions of the tagged materials as well as the variation of isotope concentration with depth. Because current technology allows the simultaneous detection of one or more tracer isotopes with distinct gamma ray signatures as well as the natural gamma ray background, several materials or stages of an operation can be separately tagged and subsequently evaluated with a single pass of a logging tool. For this reason, gamma ray spectroscopy has gained wide acceptance in the industry.
Commonly, gamma ray spectroscopy tools utilize a weighted least squares (WLS) algorithm in conjunction with a matrix of multipliers that is provided from previous calibration runs and which may be adjusted for each particular well. Full details on the operation of a gamma ray spectroscopy logging tool and the calculations used therein are set out in Applications of the Compensated Spectral Natural Gamma Tool, Gadeken et al., SWPLA Twenty-fifth Annual Logging Symposium, Jun. 10-13, 1984, and in Calibration and Analysis of Borehole and Formation Sensitivities for Gamma Ray Spectroscopy Measurements with Multiple Radioactive Tracers, Gadeken et al., The Log Analyst, May-June 1988, vol.29, No.3, pp.159-176, both of which are incorporated in their entireties herein.
Because the uses of gamma ray spectroscopy include evaluation of stimulation operations in wells that have previously been in operation, the presence of radioactive scale in the downhole tubing frequently complicates the measurement and analysis of tracer radiation. Radioactive scale typically forms on tubulars in the hole as a result of the reduction in temperature and pressure associated with production. Prior to drilling and production of a well, radium isotopes resulting from natural decay processes (.sup.226 Ra and .sup.228 Ra) and their daughter elements exist in solution in the formation water at the elevated temperature and pressure. Production of the well decreases the temperature and pressure on the fluids in the formation, causing many of the dissolved minerals to precipitate out. The material deposited on the downhole equipment as a result of this precipitation is called scale. The scale accumulated in production equipment typically contains radium co-precipitated in barium sulfate (BaSO.sub.4).
Scales accumulate in tubing, separators, and other equipment. They contain a range of Ra.sup.226 and Ra.sup.228 concentrations, from background levels to several thousand pCi/gram (several tens of decays per minute per gram). Ra.sup.226 forms about 78% of the total radium present as radioactive scale, while Ra.sup.228 comprises about 22% of the total. Scale typically occurs as a very hard, monolithic precipitate, with bulk densities that range from 2-3 g/cm.sup.3. If scale is removed in a disposal process, its bulk density may fall to around 1.6 g/cm.sup.3, due to its high porosity, which averages about 45%.
Any radium present in scale downhole will emit gamma rays that will affect attempts to accurately measure the emission of gamma rays by the radioactive tracers in the hole. If the scale contains a significant amount of radium, the presence of scale on tubing can skew the results of gamma ray logging. In many instances, this effect can be compensated for by performing a "before log" on the hole. The measurements made during the before log are then subtracted from the log made following placement of the tracer-tagged material (the "after log") and the difference between the two logs is attributed to the placement and position(s) of the tracers-tagged material(s).
In many instances, however, it is impractical or uneconomical to perform a before log. In other instances, the value of the before log may be called into doubt, if it is suspected that the step(s) of placing the tracer-tagged material may itself cause movement of the scale, thereby reducing the accuracy of calculations that are based on the before log. Hence, it is desired to provide a technique for assessing the position and quantity of radioactive scale in the tubing without requiring the performance of a before log.